Considerations of energy-saving and reduction of pollution, particularly in an urban environment, are leading motor vehicle manufacturers to equip their models with an automatic stopping/restarting system, such as the system known as “stop and go”.
As the company VALEO EQUIPEMENTS ELECTRIQUES MOTEUR recalls in its patent application FR2875549, functioning of the vehicles according to the stop and go mode is enabled by means of a reversible electrical machine, or alternator-starter, which is coupled to the thermal engine, and is supplied by an inverter in starter mode.
The use of an alternator-starter in a stop and go functioning mode consists under certain conditions of giving rise to complete stoppage of the thermal engine when the vehicle itself is at a standstill, then restarting the thermal engine subsequently, as the result for example of an action by the driver interpreted as a request for restarting.
A typical stop and go situation is that of stoppage at a red light. When the vehicle stops at the light, the thermal engine is automatically stopped, then, when the light turns green, the engine is restarted by means of the alternator-starter, as a result of detection by the system that the driver has pressed the clutch pedal.
In general, the alternator-starter must produce the appropriate engine torque as soon as a stimulus occurs indicating the wish of the driver to restart or relaunch his vehicle.
It will be appreciated that this automatic restarting function carried out by an alternator-starter system is a function which must be as clear as possible for the driver of the vehicle.
In particular, the time between the starting command and the actual starting of the thermal engine by means of the alternator-starter must be as short as possible.
For this purpose, a pre-flow step is carried out during which the rotor of the alternator-starter is supplied with electricity without power being supplied to the stator.
Upon completion of the pre-flow step, power is supplied to the stator, and the alternator-starter and the alternator-starter produces torque which is great enough to allow the thermal engine to be driven rapidly beyond the compression passage point, such as to start the engine.
This reversible alternator is mostly implanted at a face of the thermal engine, on which face a crankshaft pulley of the thermal engine is present.
A belt is provided, firstly in order to drive an alternator pulley, and if applicable one or a plurality of accessories, when the thermal engine is functioning, and secondly, in order to transmit torque supplied by the reversible alternator to the crankshaft pulley during phases of restarting or relaunching.
A tensioner is also generally provided to be supported on a strand of the belt between the alternator pulley and the crankshaft pulley.
In this configuration, the transmission of substantial engine torque by the belt upon completion of the pre-flow step can damage the belt.
In order to eliminate this disadvantage, the applicant company has proposed in the international application WO2009/077687 to limit the duration of the pre-flow step.
However, it has been found that the sudden variation of torque generated acoustic effects via the tensioner which were considered detrimental by the users.
In the French patent application FR2918127, the applicant company proposed to tension the belt by applying a torque in the direction opposite that of the starting. However, this solution does not reduce the variation of torque, on the contrary, and thus does not eliminate the acoustic effects. In addition, the tensioner must be compatible.
A need consequently exists for a method and a device for starting or restarting a thermal engine which ensures a short starting or restarting time, whilst limiting the acoustic effects which are perceived to be detrimental by the user.